In general, most vinyl polymers undergo glass transition in a narrow temperature range of about 20.degree. C. and, when analyzed with a differential scanning calorimeter (hereinafter abbreviated as DSC), give a differential curve having only one peak, which corresponds to the glass transition temperature (hereinafter abbreviated as Tg) thereof. In particular, in the case of acrylic and similar films, since the polymer compositions constituting the films have a Tg of from 20 to 40.degree. C., these films have the following problems; (1) the strength thereof varies considerably with changing temperature, (2) they are susceptible to blocking at high temperatures, (3) they have poor flexibility at low temperatures, and (4) they are insufficient in tensile strength and tear strength.
Furthermore, there is recently a growing desire for a water-soluble and/or alkali-soluble vinyl polymer, because vinyl polymers having such solubility characteristics can be used in various applications.
When a film obtained from a vinyl polymer is used as a label base, there are cases where the film is required to have the property of swelling in an aqueous alkali solution.
Examples of vinyl polymers having one Tg include the (meth)acrylate/(meth)acrylic acid copolymer film disclosed in JP-A-60-155212 and JP-A-1-165685 which is soluble in aqueous alkali solutions. (The term "(meth)acrylic" and the like used herein means "acrylic or methacrylic" and the like.) (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") Further, in JP-A-5-96689 is disclosed a laminated paper obtained by laminating a paper with a layer of a plastic which consists mainly of an acrylic ester polymer comprising units derived from either an .alpha.,.beta.-unsaturated carboxylic acid or a vinyl-containing acid anhydride and is soluble in alkaline solutions. However, these film and laminated paper each has the problems (1) to (4) described above because the polymer constituting or contained in the film or laminated paper has only one Tg. A possible method for overcoming these problems is to design a polymer so as to have not one but two or more Tg values.
An acrylic resin composition having two or more Tg values is disclosed in JP-A-59-215365. The resin composition consists mainly of a mixture of an acrylic resin emulsion having a Tg of from 20 to 50.degree. C. and an acrylic resin emulsion having a Tg of from -10 to 15.degree. C. Although superior in releasability and adhesion due to these Tg values, the resin composition is not homogeneous on a molecular level because it is not a single polymer but a mixture of polymers. Consequently, films obtained from the resin composition have poor transparency, are susceptible to whitening upon stretching, and have low tear strength. In addition, the acrylic resin is neither water-soluble nor alkali-soluble.
A resin which is not a blend but a single polymer and is considered to have two or more Tg values is given in JP-A-5-179092 and JP-B-6-51860. (The term "JP-B" as used herein means an "examined Japanese patent publication.") These techniques relate to emulsion polymerization. Since the monomers usable in emulsion polymerization are generally limited to only a small part of various polymerizable monomers, polymer designs are considerably restricted and attainable film properties are also limited.
In the techniques described in JP-A-5-179092 and JP-B-6-51860, an emulsified polymer having the so-called "core-shell" structure is synthesized by a multistage emulsion polymerization process. However, when such an emulsified polymer having the "core-shell" structure is used to form a film, the cores mostly remain dispersed although the shells are usually fused to one another. As a result, the film has a heterogeneous structure comprising the cores dispersed in a continuous phase formed from the shells. Due to its heterogeneous structure, the film formed from an emulsified polymer having the "core-shell" structure generally has problems of, e.g., poor mechanical strength and susceptibility to whitening upon water absorption. The emulsified polymer shown in JP-A-5-179092 and JP-B-6-51860 is a polymer designed mainly for a coating composition to form a coating film, and is superior in properties such as mechanical strength and water resistance to conventional emulsified polymers for coating compositions. However, for use as a film where transparency is especially required such as packaging bags and labels, the emulsified polymer proposed in those references is insufficient in transparency and mechanical strength and also in resistance to whitening upon water absorption. The emulsified polymer shown in JP-A-5-179092 and JP-B-6-51860 is neither water-soluble nor alkali-soluble.
Another method for imparting both flexibility and tensile strength is to use a polymer which undergoes glass transition over a wide temperature range. A method for obtaining such a polymer is described in JP-A-50-63085; this method is generally called a power feed method. Upon analysis with a DSC, the polymer obtained by this method gives a differential curve having one broad peak. However, the vinyl polymer for films obtained by this method necessarily contains a large proportion of a polymer component having a Tg in the range of from 20 to 40.degree. C. from the standpoint of a balance among tensile strength, flexibility, etc. The polymer therefore has a problem that the mechanical strength and other properties thereof vary considerably in the temperature range where the polymer is used.
If the emulsion polymerization process described in JP-B-6-51860 is used for synthesizing a polymer having a high acid value so as to be water-soluble and/or alkali-soluble, the .alpha.,.beta.-unsaturated carboxylic acid monomer used as an essential ingredient partly dissolves into the water to undergo aqueous-solution polymerization. The aqueous-solution polymerization yields a by-product polymer in which the content of units derived from the .alpha.,.beta.-unsaturated carboxylic acid is higher than the predetermined value. As a result, the water resistance of the polymer thus yielded becomes lower than the predetermined level. In addition, the polymer containing such a large proportion of units derived from the .alpha.,.beta.-unsaturated carboxylic acid considerably differs in solubility parameter from the polymer simultaneously produced by emulsion polymerization and is hence incompatible with the polymer produced by emulsion polymerization. As a result, the final polymer product thus obtained is not homogeneous as a whole, is opaque, and has insufficient mechanical strength.
On the other hand, use of the (meth)acrylate-(meth)acrylic acid copolymer film soluble in aqueous alkali solutions as a label which is disclosed in JP-A-60-155212 and JP-A-1-165685 is advantageous in that the label dissolves away during the step of alkali washing of bottles or the like. However, before the alkali washing solution is discarded, the polymer dissolved therein should be precipitated by, e.g., neutralization and filtered off.